msgget - get a System V message queue identifier
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgget(key_t key, int msgflg);The msgget() system call returns the System V
message queue identifier associated with the value of the key
argument. It may be used either to obtain the identifier of a previously
created message queue (when msgflg is zero and key
does not have the value IPC_PRIVATE), or to create a
new set.
A new message queue is created if key has the value
IPC_PRIVATE or key isn't
IPC_PRIVATE, no message queue with the given key
key exists, and IPC_CREAT is specified in
msgflg.
If msgflg specifies both IPC_CREAT and
IPC_EXCL and a message queue already exists for
key, then msgget() fails with errno
set to EEXIST. (This is analogous to the effect of the
combination O_CREAT | O_EXCL for
open(2).)
Upon creation, the least significant bits of the argument
msgflg define the permissions of the message queue. These
permission bits have the same format and semantics as the permissions
specified for the mode argument of open(2).
(The execute permissions are not used.)
If a new message queue is created, then its associated data structure
msqid_ds (see msgctl(2)) is initialized as
follows:
msg_perm.cuid and msg_perm.uid are set to the
effective user ID of the calling process.
msg_perm.cgid and msg_perm.gid are set to the
effective group ID of the calling process.
The least significant 9 bits of msg_perm.mode are set to
the least significant 9 bits of msgflg.
msg_qnum, msg_lspid, msg_lrpid,
msg_stime, and msg_rtime are set to 0.
msg_ctime is set to the current time.
msg_qbytes is set to the system limit
MSGMNB.
If the message queue already exists the permissions are verified, and a check is made to see if it is marked for destruction.
If successful, the return value will be the message queue identifier
(a nonnegative integer), otherwise -1 with errno indicating the
error.
On failure, errno is set to one of the following values:
A message queue exists for key, but the calling process does
not have permission to access the queue, and does not have the
CAP_IPC_OWNER capability in the user namespace that
governs its IPC namespace.
IPC_CREAT and IPC_EXCL were
specified in msgflg, but a message queue already exists for
key.
No message queue exists for key and msgflg did not
specify IPC_CREAT.
A message queue has to be created but the system does not have enough memory for the new data structure.
A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI) would be exceeded.
POSIX.1-2001, POSIX.1-2008, SVr4.
The inclusion of <sys/types.h> and
<sys/ipc.h> isn't required on Linux or by any version of
POSIX. However, some old implementations required the inclusion of these
header files, and the SVID also documented their inclusion. Applications
intended to be portable to such old systems may need to include these
header files.
IPC_PRIVATE isn't a flag field but a key_t
type. If this special value is used for key, the system call
ignores everything but the least significant 9 bits of msgflg
and creates a new message queue (on success).
The following is a system limit on message queue resources affecting a msgget() call:
System-wide limit on the number of message queues. Before Linux 3.19,
the default value for this limit was calculated using a formula based on
available system memory. Since Linux 3.19, the default value is 32,000.
On Linux, this limit can be read and modified via
/proc/sys/kernel/msgmni.
Until version 2.3.20, Linux would return EIDRM for a msgget() on a message queue scheduled for deletion.
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.
msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), sysvipc(7)
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